Phase-locked loops are used in a variety of applications such as clock recovery, frequency and phase modulation, and frequency synthesizers. A voltage-controlled oscillator is a central design element of the phase-locked loop, whereby the voltage-controlled oscillator produces an output frequency proportional to its input voltage.
A typical drawback of a voltage-controlled oscillator is its uncertainty in output frequency to the applied input voltage due to integrated circuit process variations. This leads to the need for a voltage-controlled oscillator having a large gain to provide the desired frequencies. The large voltage-controlled oscillator gain also has the effect of producing a large variation in the output frequency in response to any noise in the applied input voltage, also known as phase noise. This phase noise at the voltage-controlled oscillator output is undesirable as this limits the purity of the output signal.
As noted above, a common application of voltage-controlled oscillators are within wireless communication systems. Wireless communication systems typically require frequency synthesis in both the receive path circuitry and the transmit path circuitry. For example, cellular phone standards in the United States and Europe define a cellular telephone system with communication centered in two frequency bands at about 900 MHz and 1800 MHz.
A dual band cellular phone is capable of operating in both the 900 MHz frequency band and the 1800 MHz frequency band. Within the frequency bands, the cellular standards define systems in which base station units and mobile units communicate through multiple channels, such as 30 kHz (IS-54) or 200 kHz (GSM) wide channels. For example, with the IS-54 standard, approximately 800 channels are used for transmitting information from the base station to the mobile unit, and another approximately 800 channels are used for transmitting information from the mobile unit to the base station. A frequency band of 869 MHz to 894 MHz and a frequency band of 824 MHz to 849 MHz are reserved for these channels, respectively.
Because the mobile unit must be capable of transmitting and receiving on any of the channels for the standard within which it is operating, a frequency synthesizer must be provided to create accurate frequency signals in increments of the particular channel widths, such as for example 30 kHz increments in the 900 MHz region.
Phase-locked loop circuits including voltage-controlled oscillators are often used in mobile unit applications to produce the desired output frequency. An example of a phase-locked loop circuit in mobile applications is illustrated in FIGS. 1 and 2.
FIG. 1 is a block diagram example of a receive path circuitry 150 for a prior art wireless communication device, such as a mobile unit in a cellular phone system. An incoming signal is received by the antenna 108, filtered by a band-pass filter 110, and amplified by a low noise amplifier 112. This received signal is typically a radio-frequency signal, for example a 900 MHz or 1800 MHz signal. This radio-frequency signal is usually mixed down to a desired intermediate frequency before being mixed down to baseband. Using a reference frequency (fREF) 106 from a crystal oscillator 105, frequency synthesizer 100 provides an RF mixing signal (RFOUT) 102 to mixer 114. Mixer 114 combines this RFOUT signal 102 with the filtered and amplified input signal 113 to produce a signal 115 that has two frequency components. The signal is filtered by band-pass filter 116 to provide an IF signal 117. This IF signal 117 is then amplified by variable gain amplifier 118 before being mixed down to baseband by mixers 122 and 124.
Signal processing in mobile phones is typically conducted at baseband using in-phase (I) and quadrature (Q) signals. The Q signal is offset from the I signal by a phase shift of 90 degrees. To provide these two signals, an IF mixing signal 104 and a dual divide-by-two and quadrature shift block 120 may be utilized. Frequency synthesizer 100 generates an IFOUT signal 104; for example, at about 500 MHz; that is divided by 2 in block 120 to provide mixing signals 119 and 121. Block 120 delays the signal 121 to mixer 122 by 90 degrees with respect to the signal 119 to mixer 124.
Block 120 may be implemented with two flip-flop circuits operating off of opposite edges of the signal 104, such that the output of the flip-flops are half the frequency of the signal 104 and are 90 degrees offset from each other. The resulting output signals 123 and 125 have two frequency components.
Assuming the baseband frequency is centered at DC, the signal is filtered using low-pass filters 126 and 128. The resulting baseband signal 123 is the Q signal, and the resulting baseband signal 125 is the I signal. These signals 123 and 125 may be further processed at baseband by processing block 130 and provided to the rest of the mobile phone circuitry as I and Q signals 131 and 132.
FIG. 2 is a block diagram of a prior art phase-locked loop circuitry 200 for synthesizing one of the frequencies required by frequency synthesizer 100. A second phase-locked loop circuit may be implemented to provide the second frequency.
The reference frequency 106 is received by a divide-by-R counter 204, and the output frequency 102 is received by a divide-by-N counter 214. The resulting divided signals 216 and 218 are received by a phase detector 206. The phase detector 206 determines the phase difference between the phase of the divided signal 216 and the phase of the divided signal 218. The phase detector 206 uses this phase difference to drive a charge pump 208. The charge pump 208 provides a voltage output that is filtered by a loop filter 210 to provide a voltage control signal 220. The voltage control signal 220 controls the output frequency 102 of a voltage-controlled oscillator 212.
For a typical mobile phone application, the frequency 104 will remain constant, while the frequency 102 will change depending upon the channel of the incoming signal. Thus, a first phase-locked loop may be used to provide the frequency 104, and its N and R values may be programmed once and then left alone. A second phase-locked loop may be used to provide the frequency 102, and its N and R values may be selectively programmed to provide the desired signal 102. If desired, the R value for this second phase-locked loop may be programmed once and left alone, while the N value may be used to select the desired signal 102.
The typical transmit path circuitry (not shown) for a wireless communication device, such as a mobile unit in a cellular phone system, may include circuitry to move the outgoing signal from baseband to an RF transmission frequency. A transmit frequency band for cellular phone systems typically includes the identical number of channels as included within the receive frequency band. The transmit channels, however, are shifted from the receive channels by a fixed frequency amount.
As noted above, the phase-locked loop circuitry typically utilizes a phase detector to monitor phase differences between the divided reference frequency and the divided output frequency to drive a charge pump. The charge pump delivers packets of charge proportional to the phase difference to a loop filter.
The loop filter outputs a voltage that is connected to the voltage-controlled oscillator to control its output frequency. The action of this feedback loop attempts to drive the phase difference to zero to provide a stable and programmable output frequency. The values for the reference frequency and the divider circuits may be chosen depending upon the standard under which the mobile unit is operating.
The performance of the communication system, however, is critically dependent on the purity of the synthesized high-frequency output signals. For signal reception, impure frequency sources result in mixing of undesired channels into the desired channel signal. For signal transmission, impure frequency sources create interference in neighboring channels and limit a receivers ability to recover the transmitted data.
A frequency synthesizer, therefore, must typically meet very stringent requirements for spectral purity. The level of spectral purity required in cellular telephone applications makes the design of a phase-locked loop frequency synthesizer solution quite demanding.
Three types of spectral impurity will typically occur in voltage-controlled oscillator circuits that are used in phase-locked loop implementations for frequency synthesis: harmonic distortion terms associated with output frequency, spurious tones near the output frequency, and phase noise centered on the output frequency.
Generally, harmonic distortion terms are not too troublesome because harmonic distortion terms occur far from the desired fundamental and harmonic distortion terms' effects may be eliminated in cellular phone circuitry external to the frequency synthesizer.
Spurious tones, however, often fall close to the fundamental. Spurious tones, including reference tones, may be required by a cellular phone application to be less than about −70 dBc, while harmonic distortion terms may only be required to be less than about −20 dBc. It is noted that the “c” indicates the quantity as measured relative to the power of the “carrier” frequency, which is the output frequency.
Phase noise is undesired energy spread continuously in the vicinity of the output frequency. Phase noise can be the most damaging of the three to the spectral purity of the output frequency.
The phase-locked loop bandwidth has a strong impact on both phase-locked loop noise and on phase-locked loop settling time. In general, a wider bandwidth will lead to faster settling but will result in higher noise. Typically the phase-locked loop bandwidth can vary by +/−80% or more due to integrated circuit component tolerances. In turn, the varying of the phase-locked loop bandwidth causes less control in phase-locked loop settling time and in phase-locked loop noise.
Therefore, it is desirable to integrate a phase-locked loop with a voltage-controlled oscillator that provides a reduced variation in the phase-locked loop bandwidth. Moreover, it is desirable to provide an integrated phase-locked loop and a voltage-controlled oscillator, which enables a quick set-time of the phase-locked loop bandwidth. Lastly, it is desirable to provide an integrated phase-locked loop and a voltage-controlled oscillator that is capable of setting a phase-locked loop bandwidth quickly using only an external frequency reference.